Nonvolatile semiconductor memory device and method for manufacturing the same

ABSTRACT

A nonvolatile semiconductor memory device includes memory cell transistors, peripheral transistors, first post-oxidation films provided on the gate electrode of all of the memory cell transistors, second post-oxidation films provided on the gate electrode of all of the peripheral transistors, first insulating films provided on the first post-oxidation films and covering a side surface of the gate electrode of all of the memory cell transistors and second insulating films provided on the second post-oxidation films and covering a side surface of the gate electrode of all of the peripheral transistors. The first and second insulating films are harder for an oxidizing agent to pass therethrough than a silicon oxide film, and the first and second insulating films are oxidized.

This application division of the earlier filing date of co-pending U.S.Pat. application Ser. No. 09/556,777, filed on Apr. 25, 2000, andJapanese Patent Application No. 11-118115, filed Apr. 26, 1999. Theentire contents of those applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

This invention relates to a nonvolatile semiconductor memory device anda method for manufacturing the same.

As is well known in the art, a semiconductor memory has cell transistorsand peripheral transistors formed on the same substrate. As one examplethereof, an electrically erasable and programmable read only memory inwhich data erasing and programming can be electrically effected is wellknown.

FIG. 1 shows an EEPROM. That is, FIG. 1 schematically shows theconstruction of cell transistors (including selection gate transistors)and peripheral transistors of a conventional NAND type EEPROM.

The construction of the cell transistor and peripheral transistor of theNAND type EEPROM is explained below according to the manufacturingprocess thereof.

FIGS. 2A to 2D show the manufacturing process of the cell transistorsand peripheral transistors of the conventional NAND type EEPROM.

First, as shown in FIG. 2A, for example, after a well region and elementisolation region (neither of them is shown in the drawing) are formed inthe surface area of a silicon substrate 101, a thermal oxidation film102 used as a gate insulating film or tunnel oxide film is formed on thewell region.

Then, in the memory cell region, gate electrodes 103 of stacked gatestructure are formed on the thermal oxidation film (tunnel oxide film)102 and, in the peripheral circuit region, gate electrodes 104 ofsingle-layered structure are formed on the thermal oxide film (gateinsulating film) 102.

The gate electrode 103 in the memory cell region has a well knownstructure in which, for example, a control gate electrode 103 c isstacked on a floating gate 103 a used as a charge storing layer while anONO film (oxide film/nitride film/oxide film) 103 b used as aninter-gate insulating film is disposed therebetween.

Next, as shown in FIG. 2B, post-oxidation films 105 for restoring thegate electrodes 103, 104 from the processing damage are formed.

Then, as shown in FIG. 2C, impurity 106 is implanted to form source anddrain diffusion regions of the respective transistors.

After this, as shown in FIG. 2D, the implanted impurity 106 is activatedby annealing and driven towards the channel region side to form sourceand drain diffusion layers 106′.

Next, after an inter-level insulating film 107 is formed on thestructure, contacts 108 and inter-connection layers 109 connected to theelectrodes 104 and contacts 110 and bit lines 111 connected to thesource/drain diffusion layers 106′ are formed to form the celltransistors and peripheral transistors of the structure shown in FIG. 1.

However, if the conventional cell transistors and peripheral transistorsare formed as described above, the length of the overlap area of thesource/drain diffusion layer 106′ over the gate electrode 103 or 104varies depending on the condition of the annealing process effectedafter the impurity 106 is implanted.

For example, if the annealing process is not sufficiently effected andthe source/drain diffusion layer 106′ does not overlap and is offsetfrom the gate electrode 103 or 104, the offset portion acts as aparasitic resistor to prevent a sufficiently large drain current fromflowing in the device.

On the other hand, if the annealing process is excessively effected andthe source/drain diffusion layer 106′ extends deeply into the channelregion, the short channel effect becomes significant and thesource-drain withstand voltage is lowered, thereby degrading the devicecharacteristic.

Generally, the gate length in the memory cell is shorter than that inthe peripheral transistor. Therefore, the short channel effect in thememory cell tends to become more noticeable. That is, if the annealingprocess is sufficiently effected for the peripheral transistor, thereoccurs a possibility that punch through may occur in the cell transistorand selection transistor.

In the case of NAND type EEPROM, since the source and drain diffusionlayers 106′ of the memory cells are satisfactory if they canelectrically connect the cells which are serially arranged, it is notnecessary to overlap the source/drain diffusion layer 106′ over the gateelectrode 103. That is, it can be the that the annealing process afterthe impurity 106 is implanted is effected to the least possible degreefrom the viewpoint of the characteristic of the cell transistor andselection transistor.

Further, in the case of the post-oxidation amount after the gateprocessing, the post-oxidation for sufficiently compensating for theprocessing damage is necessary, but the post-oxidation increases thebird's beak amount. In a case where the memory cell has a short gate, anincrease in the bird's beak amount by the post-oxidation (refer to aportion A in FIG. 1, for example) lowers the coupling ratio, degradesthe programming and erasing characteristics and is not preferable.

In the case of the peripheral transistor, since the gate is relativelylong, it is permitted to sufficiently effect the post-oxidation (referto a portion B in FIG. 1, for example).

Thus, since the NAND type EEPROM includes transistors having differentgate lengths and the post-oxidation amount and the most suitableannealing condition for impurity diffusion are different depending onthe gate lengths of the transistors, the difference causes a main factorwhich lowers the process margin.

BRIEF SUMMARY OF THE INVENTION

A nonvolatile semiconductor memory device according to an aspect of theinvention comprises: a semiconductor substrate having a peripheralcircuit region and a memory cell region; a plurality of erasable andprogrammable memory cell transistors each having a gate electrode andprovided in the memory cell region; a plurality of peripheraltransistors each having a gate electrode and provided in the peripheralcircuit region; first post-oxidation films each provided on the gateelectrode of all of the plurality of erasable and programmable memorycell transistors; second post-oxidation films each provided on the gateelectrode of all of the plurality of peripheral transistors; firstinsulating films each provided on the first post-oxidation films andcovering a side surface of the gate electrode of all of the plurality oferasable and programmable memory cell transistors, the first insulatingfilms being harder for an oxidizing agent to pass therethrough than asilicon oxide film, and the first insulating films being oxidized; andsecond insulating films each provided on the second post-oxidation filmsand covering a side surface of the gate electrode of all of theperipheral transistors, the second insulating films being harder for anoxidizing agent to pass therethrough than a silicon oxide film, and thesecond insulating films being oxidized.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a cross sectional view showing a conventional NAND typeEEPROM;

FIGS. 2A, 2B, 2C and 2D are cross sectional views showing theconventional NAND type EEPROM in the respective main manufacturingsteps;

FIG. 3A is a plan view showing a NAND type EEPROM according to a firstembodiment of this invention;

FIG. 3B is a cross sectional view taken along the 3B—3B line of FIG. 3A;

FIG. 4 is a circuit diagram showing an equivalent circuit of the NANDtype EEPROM;

FIGS. 5A, 5B, 5C and 5D are cross sectional views showing the NAND typeEEPROM according to the first embodiment of this invention in therespective main manufacturing steps;

FIG. 6 is a cross sectional view showing a first modification of theNAND type EEPROM according to the first embodiment of this invention;

FIG. 7 is a cross sectional view showing a second modification of theNAND type EEPROM according to the first embodiment of this invention;

FIG. 8 is a cross sectional view showing a third modification of theNAND type EEPROM according to the first embodiment of this invention;

FIGS. 9A and 9B are cross sectional views each showing one example offormation of a contact hole;

FIGS. 10A and 10B are cross sectional views each showing another exampleof formation of a contact hole;

FIG. 11 is a plan view showing a fourth modification of the NAND typeEEPROM according to the first embodiment of this invention;

FIG. 12 is a cross sectional view showing a fifth modification of theNAND type EEPROM according to the first embodiment of this invention;

FIG. 13 is a cross sectional view showing a NAND type EEPROM accordingto a second embodiment of this invention;

FIGS. 14A, 14B, 14C and 14D are cross sectional views showing the NANDtype EEPROM according to the second embodiment of this invention in therespective main manufacturing steps;

FIG. 15 is a diagram showing the characteristic of the NAND type EEPROMaccording to the second embodiment of this invention in comparison withthe characteristic of the conventional NAND type EEPROM;

FIG. 16A is a circuit diagram showing an equivalent circuit of an ANDtype EEPROM; and

FIG. 16B is a circuit diagram showing an equivalent circuit of a NORtype EEPROM.

DETAILED DESCRIPTION OF THE INVENTION

There will now be described embodiments of this invention with referenceto the accompanying drawings. In the present embodiments, for example, aNAND type EEPROM is used as a nonvolatile semiconductor memory device.

First Embodiment

FIG. 3A is a plan view showing the schematic construction of a NAND typeEEPROM according to a first embodiment of this invention and FIG. 3B isa cross sectional view taken along the 3B—3B line of FIG. 3A.

As shown in FIGS. 3A, 3B, the NAND type EEPROM has a memory cell region(cell array) 12 and peripheral circuit region 13 formed on a siliconsubstrate 11, for example.

In the memory cell region 12, memory cell transistors, selection gatetransistors and the like are formed. In this embodiment, transistorsformed in the memory cell region 12 are generally called celltransistors.

Further, in the peripheral circuit region 13, transistors constituting amemory core circuit including a row decoder, column decoder, senseamplifiers and the like and transistors constituting an I/O circuit areformed. In this embodiment, transistors formed in the peripheral circuitregion 13 are generally called peripheral transistors.

An element isolation region 12 b is formed on the surface of the siliconsubstrate 11 in the memory cell region 12. The element isolation regions12 b separate a plurality of stripe-form element regions 12 a formed inthe memory cell region 12 and extending in the column direction. In theelement regions 12 a, for example, the surface of a p-type well regionformed on the silicon substrate 11 is exposed.

In part of the element regions 12 a, n-type source diffusion layers 21 aare formed, and in another part of the element regions 12 a, n-typedrain diffusion layers 21 b are formed. Between the source diffusionlayer 21 a and the drain diffusion layer 21 b, eighteen cell transistorsare formed. The eighteen cell transistors are serially connected.

The cell transistor among the eighteen cell transistors which isconnected to the source diffusion layer 21 a is a source side selectiongate transistor SGS and the cell transistor connected to the draindiffusion layer 21 b is a drain side selection gate transistor SGD. Theremaining sixteen cell transistors except the above two cell transistorsare memory cell transistors ST. The sixteen memory cell transistors STserially connected to one another constitute one unit cell (NAND cell).

Each of the memory cell transistors ST includes a gate oxide film 31,gate electrode 35 and source and drain diffusion layers 21 formed in theelement region 12 a.

The gate electrode 35 of the memory cell transistor ST of this examplehas a stacked gate structure. The stacked gate structure is constructedby a floating gate 32 formed on the gate oxide film 31, an inter-gateinsulating film 33 formed on the floating gate 32, and a control gate 34formed on the inter-gate insulating film 33. For example, the gate oxidefilm 31 is formed by oxidizing the surface of the substrate 11 which isexposed in the element region 12 a. In a NAND type EEPROM using a tunnelcurrent at the data programming/erasing, the gate oxide film 31 is alsocalled a tunnel oxide film. The floating gate 32 stores charges(generally, electrons) to control the threshold voltage of the memorycell transistor ST and is also called a charge storage layer. Theinter-gate insulating film 33 isolates the floating gate 32 and controlgate 34 from each other and is formed of a silicon oxide film/siliconnitride film/silicon oxide film (ONO film), for example. The controlgates 34 on the same row are connected to a corresponding one of wordlines WL0 to WL15 for selecting the row of the cell array.

The drain side selection gate transistor SGD of this example hassubstantially the same structure as the memory cell transistor ST exceptthat one of the source and drain diffusion layers 21 is formed of thedrain diffusion layer 21 b.

Likewise, the source side selection gate transistor SGS of this examplehas substantially the same structure as the memory cell transistor STexcept that one of the source and drain diffusion layers 21 is formed ofthe source diffusion layer 21 a.

The outer surface of the gate electrode 35 is covered with a firstinsulating film 37 formed of a silicon nitride (SiN) film with apost-oxidation film 36 disposed therebetween. That is, the firstinsulating film 37 is selectively formed only on the memory cell region12 so as to cover all of the cell transistors ST, SGS, SGD.

An inter-level insulating film 38 is formed on the first insulating film37. In the inter-level insulating film 38, contacts 39 b and 39 apassing through a thermal oxide film (in this example, which is formedof the same thermal oxide film as the gate oxide film 31) formed on theelement regions 12 a and the first insulating film 37 and respectivelyreaching the drain diffusion layers 21 b and source diffusion layers 21a are formed.

On the inter-level insulating film 38, bit lines (BL1, BL2, . . . ) 40connected to the drain diffusion layers 21 b via the contacts 39 b areformed in the column direction. In the inter-level insulating film 38,source lines (SL) connected to the source diffusion layers 21 a via thecontacts 39 a are formed in a row direction perpendicular to the columndirection. Thus, a memory cell array of the NAND type EEPROM shown inFIG. 4 is constructed.

As shown in FIG. 3B, each of the peripheral transistors CT formed in theperipheral circuit region 13 includes a gate oxide film 31, gateelectrode 41 and source/drain diffusion layers 42, 43.

The gate electrode 41 of the peripheral transistor CT of this examplehas a gate length larger than that of the gate electrode 35 of the celltransistors ST, SGD, SGS. The structure thereof is not the stacked gatestructure but is a general gate structure having a single-layered gateelectrode. The general gate structure is hereinafter referred to as asingle-gate structure for convenience in this specification.

Further, the outer surface of the gate electrode 41 is covered with onlya post-oxidation film 36.

An inter-level insulating film 38 is formed on the post-oxidation film36. Contacts 44 each passing through the post-oxidation film 36 andreaching the gate electrode 41 are formed in the inter-level insulatingfilm 38.

Interconnection layers 45 each connected to a corresponding one of thegate electrodes 41 via the contact 44 are formed on the inter-levelinsulating film 38.

Next, one example of a manufacturing method of the NAND type EEPROMaccording to the first embodiment is explained.

FIGS. 5A, 5B, 5C and 5D are cross sectional views showing the NAND typeEEPROM according to the first embodiment of this invention in therespective main manufacturing steps.

First, as shown in FIG. 5A, an element isolation region 12 b is formedon the surface of a silicon substrate 11 to isolate element regions 12a. Since the cross section of FIG. 5A is taken along the element region12 a, the element isolation region 12 b is not shown in FIG. 5A. Then, aportion of the substrate 11 (or well region) exposed in the elementregion 12 a is subjected to the thermal oxidation process to form athermal oxidation film. The thermal oxidation film is used as a gateinsulating film 31. Next, gate electrodes 35 of stacked gate structureare formed on the gate oxide film 31 in the memory cell region 12 andgate electrodes 41 of single-gate structure are formed on the gate oxidefilm 31 in the peripheral circuit region 13. The above gate electrodesare both formed to cross the element regions 12 a, for example. The gateelectrodes 35 of stacked gate structure can be formed by use of a wellknown method. As one example of the method, a method for forming afloating gate 32 on the gate oxide film 31, forming an inter-gateinsulating film 33 on the floating gate 32 and forming a control gate 34on the inter-gate insulating film 33 is given. After this, the surfacesof the gate electrodes 35, 41 are subjected to the oxidation process.The oxidation process is effected to compensate for the processingdamage of the gate electrodes 35, 41. As a result, post-oxidation films36 are formed on the surfaces of the gate electrodes 35, 41. Then,impurity 21′ is ion-implanted into the element regions 12 a with thegate electrodes 35, 41 and element isolation region 12 b used as a mask.The impurity 21′ is ion-implanted to form diffusion layers 21, 21 a, 21b, 42, 43 of transistors (ST, SGS, SGD, CT).

Next, as shown in FIG. 5B, a first insulating film 37 formed of asilicon nitride film is deposited on the structure shown in FIG. 5A. Thefirst insulating film 37 is not limited to the silicon nitride film anda film which makes it difficult for an oxidizing agent (oxidizing seed)to pass therethrough at the later annealing time in an oxidationatmosphere may be used.

After this, as shown in FIG. 5C, the first insulating film 37 formed onthe peripheral circuit region 13 is removed. When the above removingprocess is effected, a photoresist pattern covering the memory cellregion 12 or a photoresist pattern having an window corresponding to theperipheral circuit region 13 is formed by use of the photolithographytechnology, for example. Then, the first insulating film 37 may beremoved by effecting a CDE (Chemical Dry Etching) method using the abovephotoresist pattern as a mask.

Next, as shown in FIG. 5D, the doped impurity 21′ is activated byannealing in the oxidation atmosphere. As a result, the doped impurity21′ is diffused in the depth direction of the substrate 11 and in thelateral direction towards a portion below each of the gate electrodes35, 41. Thus, diffusion layers 21, 21 a, 21 b, 42, 43 are formed.

As described above, the structure having the memory cell region 12covered with the first insulating film 37, that is, the structure shownin FIG. 5C is subjected to the annealing process in the oxidationatmosphere. At this time, since the first insulating film 37 is notformed on the surface of the peripheral circuit region 13, a largeramount of oxidizing agent reaches the silicon substrate 11 in theperipheral circuit region 13 than in the memory cell region 12.Therefore, the diffusion speed of the impurity 21′ in the peripheralcircuit region 13 is increased so that the source/drain diffusion layers42, 43 will sufficiently overlap the gate electrode 41.

Since the memory cell region 12 is covered with the first insulatingfilm 37, almost no oxidizing agent reaches the silicon substrate 11 inthe memory cell region 12 even if the structure is subjected to theannealing process in the oxidation atmosphere. Therefore, the impurity21′ is not so diffused as in the peripheral transistor CT, therebymaking it possible to suppress the short channel effect.

In a case where tungsten silicide (WSi) is used for the control gate 34,it is considered that abnormal oxidation of WSi will occur due to theannealing process effected in the oxidation atmosphere. The abnormaloxidation of WSi tends to occur in a portion where the gate length ofthe cell transistor ST is small, for example.

However, in the first embodiment in which the memory cell region 12 iscovered with the first insulating film 37, the abnormal oxidation of WSican be suppressed even if WSi is used for the control gate 34 since theoxidizing agent can be suppressed from reaching the gate electrode 35.

Further, the bird's beak amount for the gate insulating film 31 in thememory cell region 12 and the post oxidation amount on the side wall ofthe gate electrode 35 can be reduced by leaving the first insulatingfilm 37 on the memory cell region 12 in comparison with a case where thefirst insulating film 37 is removed. Reductions in the bird's beakamount and post oxidation amount are particularly effective forsuppressing a lowering in the ratio (coupling ratio) of the capacitancebetween the control gate 34 and the floating gate 32 to the capacitancebetween the floating gate 32 and the substrate 11.

The post oxidation amount can be changed according toformation/non-formation of the first insulating film 37 for theperipheral transistor CT for which it is desired to sufficiently effectthe post oxidation so as to compensate for the processing damage and forthe cell transistors ST, SGS, SGD for which it is not desired toexcessively effect the post oxidation.

Next, after an inter-level insulating film 38 is formed on the structureshown in FIG. 5D, contacts 44 and interconnection layers 45 connected tothe gate electrodes 41 are formed, bit lines 40 and contacts 39 bconnected to the drain diffusion layers 21 b are formed and source linesand contacts 39 a connected to the source diffusion layers 21 a areformed. As a result, a NAND type EEPROM of a structure shown in FIGS.3A, 3B is completed.

In the first embodiment, the peripheral circuit region 13 can beselectively subjected to the oxidation process.

That is, the annealing process is effected in the oxidation atmospherewith the memory cell region 12 covered with the first insulating film37. Therefore, even in a case where the gate lengths of transistors aredifferent, it is possible to simultaneously satisfy the annealingcondition for impurity diffusion and the post oxidation amount. As aresult, reductions in the process margin due to a difference in theoptimum annealing condition for impurity diffusion and the postoxidation amount depending on the gate lengths of the transistors can besuppressed and this invention is extremely useful for attaining the highperformance of the device.

First Modification of First Embodiment

FIG. 6 is a cross sectional view showing a first modification of theNAND type EEPROM according to the first embodiment of this invention.

As shown in FIG. 6, when the first insulating film 37 is removed, it isnot necessary to remove the first insulating film for all of theperipheral transistors CT. That is, the first insulating film 37 may beremoved only for the peripheral transistors CT in which it is desired tocause source/drain layers 42-1, 43-1 to sufficiently overlap a gateelectrode 41-1 or the peripheral transistors CT in which it is desiredto sufficiently effect the post-oxidation process.

Second Modification of First Embodiment

FIG. 7 is a cross sectional view showing a second modification of theNAND type EEPROM according to the first embodiment of this invention.

As shown in FIG. 7, like the structure of the gate electrode 35 of thecell transistor (ST, SGD, SGS), a gate electrode 41′ of each of theperipheral transistors CT can be formed with a stacked gate structure.In this case, it is sufficient if the interconnection layer 45 iselectrically connected to at least the first-layered gate electrode 32as is well known in the art.

Further, in the second modification, an inter-gate insulating film 33can be formed in the gate electrode 41′ of the peripheral transistor CT.Therefore, the bird's beak amount for the inter-gate insulating film 33can be made different in an area where the first insulating film 37 isleft behind and in an area where it is removed.

Third Modification of First Embodiment

FIG. 8 is a cross sectional view showing a third modification of theNAND type EEPROM according to the first embodiment of this invention.

As shown in FIG. 8, a gate electrode 35′ of the selection gatetransistor SGD (SGS) can be formed with a structure different from thestructure of the gate electrode 35 of the memory cell transistor ST.

In this example, the inter-gate insulating film 33 is removed from thegate electrode 35′ of the selection gate transistor SGD.

A gate electrode 41″ of the peripheral transistor CT can also be formedwith the same structure as the gate electrode 35′ of the selection gatetransistor SGD.

Fourth Modification of First Embodiment

FIGS. 9A and 9B are cross sectional views showing one example offormation of contact hole.

As shown in FIG. 9A, in the NAND type EEPROM according to thisinvention, the first insulating film 37 is formed on the elementisolation region 12 b and diffusion layer 21 b. In this case, a materialconstituting the inter-level insulating film 38 and a materialconstituting the first insulating film 37 are made different from eachother. Further, as an etchant used for an RIE (Reactive Ion Etching)process for forming a contact hole 39 b, an etchant which easily etchesthe inter-level insulating film 38 and is difficult to etch the firstinsulating film 37 is used. Thus, the etching process can be temporarilystopped at the first insulating film 37.

By thus temporarily stopping the etching process at the first insulatingfilm 37, the etching process is suppressed from acting on the elementisolation region 12 b even if the formation position of the contact hole39 b extends over the element isolation region 12 b because of themisalignment of the mask, for example.

After the first contact hole 39 b is formed in the inter-levelinsulating film 38, as shown in FIG. 9B, an etchant used for the RIE(Reactive Ion Etching) process is changed to an etchant which easilyetches the first insulating film 37 and is difficult to etch the elementisolation region 12 b. Then, the first insulating film 37 is etched toform a second contact hole 39 b′ in the first insulating film 37. As aresult, the contact holes 39 b, 39 b′ reaching the drain diffusion layer21 b, for example, are formed in the inter-level insulating film 38.

With the above contact forming method, the etching process is suppressedfrom acting on the element isolation region 12 b even if the formationposition of the contact hole 39 b extends over the element isolationregion 12 b. Therefore, it is prevented that the element isolationregion 12 b is etched and a hole exceeding over the pn junction betweenthe diffusion layer 21 b and the substrate 11 is formed. If the abovehole is formed, for example, the bit line BL extends over the diffusionlayer 21 b and is brought into contact with the substrate 11. As aresult, a junction leak will increase.

However, in the EEPROM according to this invention, the etching processcan be suppressed from acting on the element isolation region 12 b evenif the formation position of the contact hole 39 b extends over theelement isolation region 12 b. Therefore, an advantage that an increasein the junction leak can be suppressed can be attained.

The contact hole 39 b may have a diameter larger than the width of theelement region as shown in FIGS. 10A and 10B. In this case, the contacthole 29 b overlaps the element isolation region 12 b.

Thus, an increase in the junction leak can be suppressed in the EEPROMhaving the first insulating film 37 in the memory cell region 12. It istherefore preferable to leave the first insulating film 37 on that partof the diffusion layer in which a contact hole is to be made and anearby part of the diffusion layer, while any other part of the film 37is removed after the annealing is effected in the oxidation atmosphere.FIG. 11 is a plan view of an EERROM of this type, which is the fourthembodiment of the invention.

Fifth Modification of First Embodiment

FIG. 12 is a cross sectional view showing a fifth modification of theNAND type EEPROM according to the first embodiment of this invention.

In the first embodiment, the first insulating film 37 is formed on thepost-oxidation film 36, but this is not limitative. For example, asshown in FIG. 12, a second insulating film 51 such as a silicon dioxidefilm (which is also called a TEOS film) which permits an oxidizing agentto pass therethrough and is formed by using material gas of TEOS (TetraEpoxy Silane) may be formed between the post-oxidation film 36 and thefirst insulating film 37.

In this case, for example, since the second insulating film 51 functionsas a stopper when the first insulating film 37 is removed, the processmargin can be enlarged.

Second Embodiment

In a case where a silicon nitride film is used as a first insulatingfilm 37, there occurs a possibility that the reliability of the tunneloxide film of a memory cell will be lowered since the silicon nitridefilm contains a relatively large amount of hydrogen and it hasrelatively large mechanical film stress.

In this case, hydrogen can be extracted from the silicon nitride filmand the film quality can be improved by subjecting the structure to theannealing process after deposition of the silicon nitride film in theoxidation atmosphere. Therefore, the effect for suppressing a loweringin the reliability of the tunnel oxide film of the memory cell can besufficiently expected.

FIG. 13 is a cross sectional view showing a NAND type EEPROM accordingto a second embodiment of this invention. Further, FIGS. 14A, 14B, 14C,14D are cross sectional views showing the NAND type EEPROM of the secondembodiment of this invention in the respective main manufacturing steps.

The second embodiment will be explained together with the manufacturingmethod thereof.

First, as shown in FIG. 14A, a silicon substrate 11 (or well region) issubjected to the thermal oxidation process to form a gate oxide film(tunnel oxide film) 31 by the same method as that explained withreference to FIG. 5A. Then, gate electrodes 35 of stacked gate structureare formed on the gate oxide film 31 in a memory cell region 12 and gateelectrodes 41 of single-gate structure are formed on the gate oxide film31 in a peripheral circuit region 13. After this, the surfaces of thegate electrodes 35, 41 are subjected to the oxidation process. Theoxidation process is effected to compensate for the processing damage ofthe gate electrodes 35, 41, and as the result of the oxidation process,post-oxidation films 36 are formed on the surfaces of the gateelectrodes 35, 41. Next, impurity 21′ is ion-implanted into the siliconsubstrate 11 (or well region) with the gate electrodes 35, 41 and theelement isolation region 12 b used as a mask.

As shown in FIG. 14B, a silicon nitride film 37 is deposited on thestructure shown in FIG. 14A by the same method as explained withreference to FIG. 5B. It is desired that the silicon nitride film 37have a thickness of 50 nm or less.

Then, as shown in FIG. 14C, the doped impurity 21′ is activated byeffecting the annealing process in an oxidation atmosphere. In thiscase, the silicon nitride film 37 is subjected to the oxidation processto form a surface oxide film 37′. The surface oxide film 37′ is soformed as to have a thickness of, for example, 1 nm to 10 nm. It isdesired that the surface region of the silicon nitride film 37 beoxidized by strong oxidation, such as pyrogenic oxidation, water-vaporoxygen oxidation, ozone oxidation or oxygen-radical oxidiation. This isbecause strong oxidation can efficiently extract hydrogen from thesilicon nitride film 37.

The silicon nitride film 37 on which the surface oxide film 37′ isformed has a concentration gradient in which the hydrogen concentrationgradually becomes higher from the surface side.

Thus, an influence by the hydrogen in the silicon nitride film 37 on thegate oxide film (tunnel oxide film) 31 is reduced and then the impurity21′ is laterally diffused towards portions below the gate electrodes 35,41.

As a result, as shown in FIG. 14D, diffusion layers 21, 21 a, 21 b, 42,43 are formed.

Next, after an inter-level insulating film 38 is formed on the structureshown in FIG. 14D, contacts 44 and interconnection layers 45 connectedto the gate electrodes 41 are formed, contacts 39 b and bit lines 40connected to the drain diffusion layers 21 b are formed, and contacts 39a and source lines connected to the source diffusion layers 21 a areformed. Thus, a NAND type EEPROM with the construction shown in FIG. 12is completed.

For example, as shown in FIG. 15, the hydrogen concentration of thesilicon nitride film can be lowered and an electron trap amount dVg inthe gate oxide film (tunnel oxide film) 31 can be reduced by forcedlyforming the surface oxide film 37′ on the surface of the silicon nitridefilm 37.

That is, if the surface of the silicon nitride film 37 is subjected tothe oxidation process before deposition of the inter-level insulatingfilm 38, the hydrogen concentration of the silicon nitride film 37 canbe lowered and the hydrogen concentration of the thermal oxide film 31can be lowered. As a result, the electron trap amount dVg in the gateoxide film (tunnel oxide film) 31 can be reduced and the reliability ofthe tunnel oxide film can be prevented from being lowered.

The hydrogen concentration of the thermal oxide film shown in FIG. 15 isexpressed by a relative value when it is set at “1” when the surfaceoxide film 37′ is not formed.

Further, for example, the electron trap amount dVg is a differencebetween the maximum value and minimum value of the gate voltageoccurring in a period 20 seconds when a negative voltage is applied tothe gate and a D.C. constant current of approx. 0.1 A/cm² is caused toflow in the tunnel oxide film for approx. 20 seconds. In this case, thevalue of dVg becomes larger as the generation amount of electron trap inthe tunnel oxide film becomes larger.

With the above construction, the reliability of the gate oxide film(tunnel oxide film) 31 can be suppressed or prevented from being loweredeven if the silicon nitride film 37 is left behind on the memory cellregion 12.

In the second embodiment, the silicon nitride film 37 is left behind onthe peripheral circuit region 13, and it is possible to leave thesilicon nitride film at least on the memory cell region 12 as in thefirst embodiment. In this case, in addition to the effect that thehydrogen concentration of the silicon nitride film 37 can be lowered, anadvantage that the overlap amount or the like of the diffusion layer canbe made different in the peripheral transistor and in the celltransistor can be attained.

In the second embodiment, the impurity 21′ is doped into the substrate11 before the silicon nitride film 37 is formed, but this is notlimitative. For example, it is possible to dope the impurity 21′ intothe substrate 11 after the silicon nitride film 37 is formed.

The silicon nitride film 37 is not always necessary after the annealingprocess is effected in the oxidation atmosphere. Therefore, it ispossible to remove all of the silicon nitride film 37 after theannealing process.

Further, this invention is not limited to the NAND type EEPROM and canbe applied to an EEPROM other than the NAND type EEPROM, for example, anAND type EEPROM shown in FIG. 16A, NOR type EEPROM shown in FIG. 16B.

In the first and second embodiments, the post-oxidation film 36 isprovided on the upper surface of the control gate electrode 34 ofstacked gate structure or on the upper surface of the gate electrode 41of single-gate structure. Nonetheless, the control gates 34 and 41 mayeach comprise a conductive strip and either a silicon oxide film orsilicon nitride film, which is provided on the conductive strip. In thiscase, the post-oxidation film 36 is provided on only the sides of thecontrol gate electrode 34 or only the sides of the gate electrode 41.

In addition, this invention can be variously modified without departingfrom the technical scope thereof.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A nonvolatile semiconductor memory device,comprising: a semiconductor substrate having a peripheral circuit regionand a memory cell region; a plurality of erasable and programmablememory cell transistors each having a gate electrode and provided in thememory cell region; a plurality of peripheral transistors each having agate electrode and provided in the peripheral circuit region; firstpost-oxidation films each provided on the gate electrode of all of theplurality of erasable and programmable memory cell transistors; secondpost-oxidation films each provided on the gate electrode of all of theplurality of peripheral transistors; first insulating films eachprovided on the first post-oxidation films and covering a side surfaceof the gate electrode of all of the plurality of erasable andprogrammable memory cell transistors, the first insulating films beingharder for an oxidizing agent to pass there through than a silicon oxidefilm, and the first insulating films being oxidized; and secondinsulating films each provided on the second post-oxidation films andcovering a side surface of the gate electrode of all of the peripheraltransistors, the second insulating films being harder for an oxidizingagent to pass there through than a silicon oxide film, and the secondinsulating films being oxidized, wherein the first insulating films andthe second insulating films each comprise a silicon nitride film, and asurface of the silicon nitride film is oxidized.
 2. The device accordingto claim 1, wherein a thickness of an oxidized region of the siliconnitride film is not smaller than 1 nm and not larger than 10 nm.
 3. Thedevice according to claim 1, wherein the silicon nitride film containshydrogen with a concentration not larger than 3×10²¹ atom/cm³.
 4. Thedevice according to claim 1, wherein the silicon nitride film containshydrogen and a concentration of the hydrogen gradually becomes higherfrom the surface of the silicon nitride film.
 5. The device according toclaim 1, wherein the gate electrode of each of the plurality of erasableand programmable memory cell transistors and the peripheral transistorscontains a metal or a metal silicide.
 6. The device according to claim5, wherein the metal contains tungsten.
 7. The device according to claim1, wherein the gate electrode of each of the plurality of erasable andprogrammable memory cell transistors and the peripheral transistors is astacked gate structure including a floating gate and a control gate, thecontrol gate comprising a metal or a metal silicide.
 8. The deviceaccording to claim 7, wherein the metal contains tungsten.
 9. Anonvolatile semiconductor memory device, comprising: a semiconductorsubstrate having a peripheral circuit region and a memory cell region; aplurality of erasable and programmable memory cell transistors eachhaving a gate electrode and provided in the memory cell region; aplurality of peripheral transistors each having a gate electrode andprovided in the peripheral circuit region; first post-oxidation filmseach provided on the gate electrode of all of the plurality of erasableand programmable memory cell transistors; second post-oxidation filmseach provided on the gate electrode of all of the plurality ofperipheral transistors; first insulating films each provided on thefirst post-oxidation films and covering a side surface of the gateelectrode of all of the plurality of erasable and programmable memorycell transistors, the first insulating films being harder for anoxidizing agent to pass there through than a silicon oxide film, and thefirst insulating films being oxidized; and second insulating films eachprovided on the second post-oxidation films and covering a side surfaceof the gate electrode of all of the peripheral transistors, the secondinsulating films being harder for an oxidizing agent to passtherethrough than a silicon oxide film, and the second insulating filmsbeing oxidized, wherein the first insulating films and the secondinsulating films each comprise a silicon nitride film, and the siliconnitride film contains hydrogen and a concentration of the hydrogengradually becomes higher from the surface of the silicon nitride film.